1. Field of the Invention
The invention relates to the field of catalyst supports with a specific surface area of less than 90 m2/g and having a pore volume greater than 0.3 cm3/g, this pore volume having an a least bimodal distribution defining an interconnected main porosity and secondary porosity, the main porosity being of an average size greater than the secondary porosity.
The support according to the invention allows catalysts to be produced for carrying out a large range of reactions on hydrocarbon cuts.
2. Description of Related Art
It is known in catalysis that the activity and/or the selectivity are affected by increasing the residence time of products on or near the catalytic sites. There, the known problem exists of intragranular diffusion limitation which can constitute a limit to the satisfactory development of the process whereby the reagents access the reaction site.
It is known that the intrinsic kinematics of a given chemical reaction is affected to a greater or lesser extent by the rate of supply of the reagents from the immediate vicinity of the catalytic particle towards the reaction site situated inside said particle. This phenomenon of diffusion limitation, which expresses the competition between the intrinsic kinetics and the supply of the material by diffusion to the inside of the particle, depends mainly on the size of the particle and on the porosity of the particle, more specifically on the structure of this porosity.
In order to overcome this problem, i.e. to be free of the intragranular diffusion limitations, several solutions have been proposed and described in patents which we cite and comment on below.
Firstly, there may be mentioned the solution of shell deposition of a catalytic phase on a core constituted by the support. By shell deposition is meant that the catalytic phase is situated in an outer layer of the support, called shell, this shell being able to be several tens to several hundred microns thick.                The patent documents WO 02/41990, WO98/14274, U.S. Pat. No. 6,280,608, U.S. Pat. No. 6,486,370 and U.S. Pat. No. 6,177,381 describe the preparation of solids of this type called “multilayer” solids. The original feature of these solids rests essentially in the deposition in the form of a shell of a support having the required porosity on another refractory support called solid support. The deposition of this shell is carried out by using a suspension of inorganic oxides which covers the solid support. The thickness of this shell varies from 40 to 400 microns. Subsequently, metals are impregnated in or on the shell for use in the dehydrogenation of paraffins.        The patent document WO 01/15803 describes the use of a catalyst for paraffin dehydrogenation applications. The support is composed of porous alumina. On the final catalyst, the percentage of pores comprised between 60 Å and 350 Å should be greater than 75% of the total pore volume of the catalyst. The volume of the pores comprised between 60 Å and 350 Å is greater than 0.5 cm3/g, and preferably comprised between 0.6 and 0.8 cm3/g. Finally, the volume of the smallest pores (less than 60 Å) should not exceed 0.05 cm3/g.        
A grain density greater than 0.5 g/cm3 is claimed, which corresponds to that of the prior art. The technique used to measure the porosity is mercury intrusion. Moreover, the catalyst should have a specific surface area greater than 100 m2/g.                The patent U.S. Pat. No. 5,358,920 describes the use of a catalyst for the dehydrogenation of hydrocarbons. The support used is produced in a single synthesis stage by neutralization of aluminum chloride by ammonium hydroxide at 60-70° C. and at a pH comprised between 7.5 and 8.5. The resulting precipitate is resuspended to be shaped by the oil drop coagulation technique. After calcination between 600° C. and 800° C. under an air-water mixture, the support has a theta alumina crystallographic form. This support has a bimodal porous distribution, with 40% of the pore volume occupied by the pores comprised between 1000 Å and 10,000 Å. It is clear to a person skilled in the art that this one-stage synthesis process cannot result in the obtaining of a hierarchical porosity, i.e. differentiated into at least two modes.        The patent U.S. Pat. No. 5,677,260 describes the preparation and the use of a composite catalyst for the dehydrogenation of paraffins to monoolefins. The support used is in the form of beads 1.4 to 2 mm in diameter. This support is mesoporous and preferably uses a gamma alumina (60 to 80% crystallinity), with a surface area comprised between 120 m2/g and 250 m2/g, and a pore volume comprised between 1.4 and 2.5 cm3/g. With regard to the diameter of the beads and the specific surface area in question, the intragranular diffusion phenomena are probably very significant in this type of support.        The patent U.S. Pat. No. 4,914,075 describes the composition of a dehydrogenation catalyst. The support is alumina with a specific surface area comprised between 50 m2/g and 120 m2/g and apparent grain density greater than 0.5 g/cm3. No details are given about either the pore volume or the size of the pores. The description of the production of the support clearly shows that this support is produced in a single synthesis stage, followed by a calcination stage (800° C. to 1020° C.) according to the nature of the desired alumina phase (gamma, theta or alpha). According to the cited patent, it is preferable to have theta alumina and this at a level of 75% crystallites (the others being able to be gamma alumina or alpha alumina). In this patent, if several crystallographic phases of alumina can be present, there is no concept of order or of hierarchy between these phases.        The patent U.S. Pat. No. 4,672,146 describes the composition of a dehydrogenation catalyst. The support is an alumina having a surface area comprised between 5 m2/g and 150 m2/g, 18% of the total pore volume of which is associated with pores less than 300 Å in diameter, and 55% of the total pore volume is associated with pores greater than 600 Å in diameter. It should be noted that the preparation process for this support comprises a single synthesis phase usually leading to a mixture of different alumina crystallographic phases.        The patent EP 1142637 describes the preparation of a catalyst for distillate hydroconversion. The support is mixed micro- (50 to 75% by weight) and macroporous (50 to 25% by weight) alumina. The authors mean by microporous that 95% of the volume of this alumina corresponds to a pore diameter of less than 80 Å. The authors mean by macroporous that at least 70% of the volume of this alumina corresponds to a pore diameter comprised between 60 Å and 600 Å. The preparation processes cited (including that of patent EP 1142637) have recourse to the precipitation in one or several stages of a gel, followed by a shaping and a heat treatment. In this case, the micro- and the macroporosity are interwoven without any hierarchical order.        The patents EP0758919 and EP0882503 describe the preparation of high-activity catalysts. The sought applications are the operations of hydrotreatment (hydrodenitrogenation, hydrodesulfuration, hydrodemetallation, hydroconversion, hydrocracking), hydrogenation/dehydrogenation, reforming, isomerization and the Claus process. These patents are centred round a method for preparing catalysts, namely the introduction of a chelating agent before, during or after impregnation of the active phase. The influence of this chelating agent would be to create an interaction between the amorphous alumina and the active phase, this interaction being identified by the presence at the surface of a microcrystalline alumina of a size comprised between 8 Å and 25 Å, as well as gamma alumina serving as support, which has a crystallite size greater than 30 Å. The appearance of this nano-crystalline phase leads to the increase in the specific surface area as well as the appearance of a bimodal mesoporous structure comprising a first set of pores of a size less than 40 Å and a second set of pores of a size greater than 50 Å. The porosity measurements are carried out on the nitrogen desorption branch. The obtained catalyst has a surface area of at least 100 m2/g.        The article by M. Pan et al, J. Memb. Sci, 158 (1999) 235-241 (“Journal of Membrane Science”) describes the preparation of membranes of nanoporous aluminas by chemical vapor deposition (CVD). The use of such a technique leads to a homogeneous covering of the surface of the support (here an alpha alumina of 0.2-micron pore size). However, because of the technique used, the deposition does not create porosity and leads to a decrease in the specific surface area.        The patent U.S. Pat. No. 5,518,979 deals with catalyst supports based on abrasion-resistant transition alumina. The alumina support is composed of gamma and kappa alumina (with a maximum of 10% delta alumina) or theta and kappa alumina (with a maximum of 10% alpha alumina) and has either a monomodal or bimodal distribution in a range of pores of a size comprised between 100 Å and 1500 Å and having a surface area ranging from 33 m2/g to 63 m2/g (determined by mercury porosimetry). The preparation method described in this patent consists of mixing two different alumina precursors, namely an amorphous alumina precursor, leading after calcination to kappa alumina, and a precursor such as a boehmite or pseudo-boehmite leading after calcination to gamma, delta or theta aluminas.        Cini et al, in J. Memb. Sci., 55 (1991) 199 (“Journal of Membrane Science”) deal with the preparation of ceramic membranes to serve as catalyst supports. They use macroporous alpha alumina supports, with multimodal pore size distribution, since these supports are ceramic tubes comprising a first layer with an 85-nm pore diameter, a second layer with a 650-nm diameter and a third layer with a 3000-nm diameter. Deposited on this alumina is a boehmite sol, precursor of a gamma alumina which would develop a specific surface area estimated by the authors at 130 m2/g. They manage to deposit up to 9% alumina, thereby allowing an increase in the specific surface area of 1 to 13 m2/g. The deposited alumina has a pore size comprised between 2.5 and 4.5 nm. The deposited films have a thickness ranging from 3 to 57 μm, which clearly indicates that the alumina is deposited either in the porosity of the most macroporous layer or on the surface of the tubes.        The patent EP 0586745 describes the formation of a ceramic membrane achieved by a purely mesoporous (pore diameter comprised between 3 and 5 nm) and microporous (pore diameter less than 2 nm) support. This porosity is not capable of resolving the problems of intragranular diffusion limitation.        